The present invention relates to a method for preparing a stable aqueous emulsion of phenolic resin, including modified phenolic resins.
Phenolic resins are today applied to a wide range of uses in various forms such as coatings, binders for inorganic materials, binders for organic materials, compounding ingredients for rubbers, laminates, moldings, castings etc. But, before assuming their final forms, phenolic resins are handled as intermediates.
Types of intemediates known hitherto, comprise the following: an aqueous solution of lower condensate of phenolic resin, an organic solvent solution of medium or higher condensate of phenolic resin, an aqueous solution of phenolic resin containing concentrated caustic alkali, a powdered one-step phenolic resin, a powdered two-step phenolic resin. These intermediate materials generally have various kinds of defects, such as storage stability, discoloration, flammability, workability, emission of noxious substances or cutting costs and so forth. Namely, an aqueous solution of a lower condensate of phenolic resin has a short storage life because it is highly reactive, and it easily releases free phenols and free aldehydes into the air during processing because it is comparatively rich in free phenol and in free aldehyde; therefore, there becoming trouble sources for unhealthy working environment and air pollution.
An organic solution of one-step phenolic resin which consists of comparatively lower molecular weight has also defects of emitting noxious substances, of being flammable, of workability, of pricing and so forth. Or an aqueous solution of phenolic resin containing concentrated caustic alkali tends to degrade the adhered or filler, so the originally excellent properties of phenolic resin do not contribute much to the final product, and moreover the product is much discolored. In the case of powdered phenolic resin, its manufacturing cost becomes higher than other forms of phenolic resin products due not only to an additional pulverizing step added to the process, but also at the same time today, requires a large sum of expenses for improving its working environment from the standpoint of labor safety and health regulations.
Therefore, development of an emulsion type intermediate phenolic resin which is free of defects or problems for manufacturing and application has generally been much desired.
However, it has hitherto generally been recognized that phenolic resins are difficult to emulsify. Namely, for emulsification of phenolic resin, there are methods by adding casein or polyvinyl alcohol individually, or with simultaneous addition of surfactants, by sulfomethylating the resin with a sodium bisulfite, by co-condensation of the resin with urea resin, etc.; but, heretofore it has been difficult to obtain an emulsion by any known method which is good in water miscibility and stable for storage. It is a generally known exception that a phenolic resin prepared by using caustic alkali as a catalyst is comparatively easy to emulsify, but a phenolic resin prepared by using amines or acids as a catalyst is very difficult to emulsify in any case.
As a result of various studies to remove such defects described above concerning intermediate materials of phenolic resin and thereby enabling to expand their uses, the inventors succeeded in preparing an emulsion of phenolic resin which is stable for storage and also of low cost, by adding polyacrylamides to phenolic resin during the course or after the completion of the reaction of phenols with aldehydes, adding urea, melamine, etc. as a modif er if required, and reacted under reflux or at somewhat lower temperature than reflux with acid or alkaline catalyst until the mixture becomes turbid.
The main object of adding polyacrylamides in the present invention is to use them as a dispersing agent for the solid fine particles of phenolic resin, and also promoting remarkably the bonding strength by reacting part of polyacrylamides with phenolic resin at the curing stage, and, at the same time, improving the brittleness of phenolic resin by imparting flexibility.
The other object of adding polyacrylamides is to prevent the darkening of phenolic resin which is one fault of the resin when cured, and very light colored phenolic resins are obtained when using ammonia or amine catalyst.